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FreeBSD-SA-18:04.vt                                         Security Advisory
                                                          The FreeBSD Project

Topic:          vt console memory disclosure

Category:       core
Module:         vt console
Announced:      2018-04-04
Credits:        Dr Silvio Cesare of InfoSect
Affects:        All supported versions of FreeBSD.
Corrected:      2018-04-04 05:24:59 UTC (stable/11, 11.1-STABLE)
                2018-04-04 05:33:56 UTC (releng/11.1, 11.1-RELEASE-p9)
                2018-04-04 05:26:33 UTC (stable/10, 10.4-STABLE)
                2018-04-04 05:33:56 UTC (releng/10.4, 10.4-RELEASE-p8)
                2018-04-04 05:33:56 UTC (releng/10.3, 10.3-RELEASE-p29)
CVE Name:       CVE-2018-6917

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.

I.   Background

On FreeBSD 11 and later, and FreeBSD 10.x systems that boot via UEFI, the
default system video console is provided by the vt(4) driver.  The console
allows the user, including an unprivileged user, to load a font at runtime.

II.  Problem Description

Insufficient validation of user-provided font parameters can result in an
integer overflow, leading to the use of arbitrary kernel memory as glyph
data.  Characters that reference this data can be displayed on the screen,
effectively disclosing kernel memory.

III. Impact

Unprivileged users may be able to access privileged kernel data.

Such memory might contain sensitive information, such as portions of the file
cache or terminal buffers.  This information might be directly useful, or it
might be leveraged to obtain elevated privileges in some way; for example,
a terminal buffer might include a user-entered password.

IV.  Workaround

The syscons sc(4) system console is not affected by this issue and may be
used on systems that do not boot via UEFI.  To use the syscons console,
set the kern.vty tunable in /boot/loader.conf as described in sc(4), and
reboot.  No workaround is available for systems that boot via UEFI.

V.   Solution

Perform one of the following:

1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

2) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

A reboot is required after the upgrade.

3) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/SA-18:04/vt.patch
# fetch https://security.FreeBSD.org/patches/SA-18:04/vt.patch.asc
# gpg --verify vt.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/10/                                                        r331983
releng/10.3/                                                      r331984
releng/10.4/                                                      r331984
stable/11/                                                        r331982
releng/11.1/                                                      r331984
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-6917>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-18:04.vt.asc>
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